In order for cancer metastasis to occur, a tumor cell must be able to mobilize itself. This mobility allows it to invade adjacent normal tissue, leave the original tumor site, enter local vasculature, and disseminate to distant organs. We have recently published an in vivo analysis of migration which demonstrates that promoting the immobility of tumor cells through the cell surface tetraspanin CD151 prevents intravasation and subsequent metastasis (Zijlstra et al., 2008). Further biochemical analysis identified Activated Leukocyte Cell Adhesion Molecule (ALCAM) as a novel tetraspanin partner of CD151 in tetraspanin enriched microdomains (TERM). Genetic experiment confirmed that ALCAM is required for CD151 to mediate immobility. We have subsequently determined that CD151 and ALCAM form an endogenous regulatory complex that limits migration by promoting integrin-dependent adhesion via Rap1 activation. Our current observations suggest that ALCAM functions as an anchoring mechanism that recruits CD151-containing TERM to cell-cell adhesions. Stabilization of this complex via CD151 or ALCAM promotes immobility while disruption of the complex can promote invasion and metastasis. We propose to determine the molecular mechanism by which CD151/ALCAM-containing TERM can promote immobility and to investigate the contribution of this complex to metastasis of prostate cancer to the bone. This application proposes to specifically investigate the molecular mechanism by which mobility is regulated, its ability to control prostate cancer metastasis, and its relation to human disease progression and patient survival. In aim #1 we will determine the endogenous mechanism by which CD151/ALCAM promote tumor cell immobility in vitro and in vivo. Orthotropic prostate metastasis models will be used in aim #2 to investigate the ability of the endogenous ALCAM/CD151 regulatory complex to influence prostate cancer metastasis. Furthermore, the correlation between ALCAM/CD151 related expression profiles and disease progression with survival within patient populations will be investigated in aim #3 The findings of the proposed work will not only shed light on the molecular mechanism of CD151 and its newly identified membrane partner ALCAM/CD166. It will also provide a critical evaluation of tumor cell immobilization as a therapeutic target in preventing dissemination and identifying potentially novel clinical targets for the treatment of invasive disease.